Microcrystal laser for generating laser pulses

ABSTRACT

A microcrystal laser for generating laser pulses has a laser resonator which has a laser medium arranged between two mirrors; and an arrangement for stabilizing the optical path length is provided. The laser resonator has a saturable absorber medium for pulse generation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of German application no. 10 2010013 766.9, filed Mar. 31, 2010, and this application claims the priorityof German application no. 10 2010 050 860.8, filed Nov. 9, 2010, andeach of which is incorporated herein by reference.

The invention relates to a microcrystal laser of the type for generatinglaser pulses. More particularly, the invention relates to a microcrystallaser for generating laser pulses that has a laser resonator including alaser medium arranged between two mirrors.

BACKGROUND OF THE INVENTION

Different types of monofrequency microcrystal lasers are known. Afeature of such lasers is the formation of one or more longitudinalmodes of the light wave field in the laser resonator. The number ofpossible longitudinal modes is defined in this way by the optical pathlength of the resonator, in which case a sufficient optical path length,one mode or a plurality of modes can develop. If the optical path lengthof the resonator is reduced, the maximum possible number of laser modesis reduced. The smaller the optical path length becomes here, the largeris accordingly the frequency spacing between two adjacent modes. Thepossible oscillating modes of such a laser are the ones which aresubjected to amplification. In order to be subjected to amplification,the modes must lie within the amplification bandwidth of the lasermedium (laser-active medium). Accordingly, with decreasing resonatorlength, the number of amplified modes decreases. The number can alsobecome zero in the case that the absolute position of the modes liesoutside of the amplification bandwidth of the laser medium. Withdecreasing resonator length, the period and the pulse duration of theselected laser pulse decreases at the same time, and in which laserpulses with pulse durations <1 ns up to <50 ps can be obtained.

By a suitable selection of the optical path length of the laserresonator it is possible that the oscillating laser mode lies in adefined manner within the amplification bandwidth, for example in themaximum, and thus is subjected to an amplification as high as possible.Accordingly, the output power of a laser is at a maximum if the opticalpath length is actively adapted to the relative position of theoscillating mode in the maximum of the amplification bandwidth of thelaser medium.

An amplitude fluctuation of a microcrystal laser in the uncontrolledrange is approximately 6% and thus is not suited for applications whichrequire a lower amplitude fluctuation from pulse to pulse. Astabilization of the optical path length of the resonator counteracts anamplitude fluctuation through smaller differences in the amplificationby centering the position of the emission wavelength in the maximum ofthe amplification bandwidth of the laser medium.

From DE 43 06 919 C2, a method for stabilizing the optical path lengthof a resonator is known. This publication discloses a microcrystal laserof the relevant type for generating laser pulses which has a laserresonator which has a laser medium (laser-active medium) arrangedbetween two mirrors. The known microcrystal laser further has anarrangement for stabilizing the optical path length of the laserresonator.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to configure a microcrystal laser in sucha manner that a monomode operation on only one laser line in theemission maximum of the amplification bandwidth of the laser medium ispossible and, at the same time, short laser pulses at low amplitudefluctuations can be generated.

This object is achieved by the microcrystal laser for generating laserpulses according to the invention which includes:

a) a laser resonator including a laser medium arranged between twomirrors;

b) an arrangement being provided for stabilizing the optical path lengthof the laser resonator; and

c) the laser resonator including a saturable absorber medium forgenerating pulses.

The fundamental idea of the invention is to provide a saturable absorbermedium in the laser resonator. It was surprisingly found in accordancewith the invention that by combining a known saturable absorber mediumwith a stabilization of the optical path length of the resonator, a lowamplitude fluctuation at short pulse duration and a monomode operationon only one laser line in the emission maximum of the amplificationbandwidth of the laser medium are possible.

Thus, the invention provides with relatively little effort amicrocrystal laser, hereinafter also referred to in short as a laser,which, with respect to the generation of short pulses with littleamplitude fluctuation, has particularly advantageous properties.

The arrangement for stabilizing the optical path length of the laserresonator can be configured according to the respective requirements.For example, and in particular, it is possible according to theinvention to thermally stabilize the optical path length of the laserresonator. This can take place according to the invention, for example,in that the pump current and/or the amplitude of the pump light ischanged so that consequently the path length of the laser resonatorchanges. Within a thermal stabilization of the path length of the laserresonator it is also possible to stabilize the laser resonator bycooling or heating the whole laser resonator or individual components ofthe laser resonator. However, according to the invention, it is alsopossible to mechanically stabilize the optical path length of the laserresonator by providing, for example, at least one piezoelectric actuatorby use of which at least one component of the laser resonator can bedisplaced. According to the invention, a combination of a thermalstabilization with a mechanical stabilization is also possible.

If the laser resonator has cooling media as a heat sink, preferablyAl₂O₃, SiC, or diamond can be used.

In principle, the saturable absorber medium can be provided at anyposition or in any component of the laser resonator.

An advantageous further embodiment of the invention provides in thisrespect that the mirror is a saturable absorber mirror which forms thesaturable absorber medium. Another advantageous further embodiment ofthe invention provides that the laser resonator is a monolithic orsemi-monolithic laser resonator.

It has been found that the favorable properties of the laser accordingto the invention can be further improved if the laser medium is cooled.An advantageous further embodiment of the invention provides in thisrespect an arrangement for dissipating heat from the laser medium.

A further embodiment of the aforementioned embodiment provides that thearrangement for dissipating heat from the laser medium has a carriermade of a material with high thermal conductivity with which the lasermedium is in a thermally conductive connection. A material of highthermal conductivity is to be understood according to the invention as amaterial, the thermal conductivity of which is higher than the one ofthe laser medium, so that an efficient dissipation of heat from thelaser medium and thus an efficient cooling of the laser medium ispossible in a particularly simple manner.

In order to prevent or at least reduce an undesired thermal interferencewith the saturable absorber medium caused by heat generated in the lasermedium, an advantageous further embodiment of the invention providesthat the saturable absorber medium is at least partially, preferablycompletely, or almost completely thermally decoupled from the lasermedium. In other words, the saturable absorber medium is one ofpartially, substantially completely, and completely decoupled from thelaser medium.

An advantageous further embodiment of the aforementioned embodimentprovides that for thermally decoupling the saturable absorber mediumfrom the laser medium, the saturable absorber medium and the lasermedium are spaced apart from each other. In this embodiment, an at leastpartial thermal decoupling of the saturable absorber medium from thelaser medium is achieved in a particularly simple manner.

In the aforementioned embodiment, a thermally insulating material can bearranged between the saturable absorber medium and the laser medium, andthe material is selected such that it does not or only to a minimalextent influences the optical properties of the laser resonator.

Advantageously, however, between the saturable absorber medium and thelaser medium, an air gap is formed as is provided by a furtherembodiment of the aforementioned embodiment. In this manner, aparticularly effective cooling of the saturable absorber medium ispossible. The air gap in a resonator, the geometrical path length ofwhich is ≦1,000 μm, can be approximately 10-500 μm. In case of laserresonators with a larger optical path length, the gap width of the airgap is to be selected according to the respective requirements.

Another advantageous further embodiment of the invention provides thatthe saturable absorber medium is arranged on a carrier element and thatbetween the carrier element and the laser medium at least one spacer isarranged. This embodiment allows via the spacer an interference with theoptical path length of the resonator. For this purpose, for example, thespacer can be configured as a piezo element.

An advantageous further embodiment of the aforementioned embodimentprovides that the material of the spacer is selected with respect to itscoefficient of thermal expansion such that during a temperature changeof the laser resonator, the optical path length of the laser resonatorchanges. If, for example, the material of the spacer has a highercoefficient of thermal expansion than the material of the saturableabsorber medium, the optical path length of the laser resonatorincreases during a temperature increase.

Another advantageous further embodiment of the invention provides acontrol device for controlling the optical path length of the laserresonator, in particular as a function of the output power of themicrocrystal laser and/or the pulse repetition rate and/or thewavelength of the laser mode. The laser can be pumped, for example, by adiode laser, the pump light of which, for example, is coupled into thelaser via a telescope. A portion of the generated laser radiation can bedirected via a beam splitter onto a photodiode which generates a signalwhich is proportional to the output power of the laser. This signal canbe converted in a controller into an actuating variable which influencesthe optical path length of the laser resonator as controlled variable.Here, for example, the current of the pump diode, the temperature of thepump diode, the temperature of the laser, the voltage at a piezo elementwhich serves as spacer between the saturable absorber medium and thelaser, and other parameters can serve as actuating variables.

According to the respective requirements, the geometrical length of thelaser resonator, and therefore the optical path length of the same, isselectable within wide limits. An advantageous further embodiment of theinvention provides in this respect that the geometrical path length ofthe laser resonator is ≦1,000 μm, in particular ≦500 μm.

In order to prevent that pump light enters into the saturable absorbermedium and causes in particular a heating of the same, anotheradvantageous further embodiment of the invention provides that the lasermedium, on its side facing the saturable absorber medium, and/or thesaturable absorber medium, on its side facing the laser medium, has acoating for reflecting the pump light back into the laser medium.

The invention is illustrated in more detail hereinafter by use of theattached drawing in which greatly schematized embodiments of a laseraccording to the invention are illustrated. Here, all featuresdescribed, illustrated in the drawing and claimed in the patent claims,individually or in any combination with each other, form the subjectmatter of the invention independent of their combination in the patentclaims and their relations and independent of their description orillustration in the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a greatly schematized side view of a first embodiment of alaser according to the invention;

FIG. 2 shows, in the same manner as FIG. 1, a second embodiment of alaser according to the invention;

FIG. 3 shows, in the same manner as FIG. 1, a third embodiment of alaser according to the invention;

FIG. 4 shows, in the same manner as FIG. 1, a fourth embodiment of alaser according to the invention;

FIG. 5 shows a diagram for clarifying the dependency of the position ofan oscillating laser mode within the amplification bandwidth; and

FIG. 6 shows a schematic block diagram of an arrangement for generatingshort laser pulses with a laser according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the figures of the drawing, identical or corresponding components areindicated with identical reference numerals.

In FIG. 1, a greatly schematized side view of a laser resonator 2 of alaser according to the invention is illustrated which has a laser medium(laser-active medium) 4 which is formed in this embodiment by a lasercrystal Nd:YVO₄. The laser medium 4 is arranged between two mirrors, inwhich the one mirror 6 in this embodiment is formed by a partlyreflective outcoupling coating. According to the invention, the laserresonator 2 has a saturable absorber medium which is formed in thisembodiment by a saturable absorber mirror 8. The embodiment illustratedin FIG. 1 relates to a monolithic structure, in which the resonatormirrors 6, 8 are provided on plane-parallel surfaces of the laser medium4.

The saturable absorber medium can be operated (e.g., driven) inreflection as well as in transmission. It can consist, for example, of asemiconductor material, for example gallium arsenide (GaAs), or a dopedcrystal, for example Cr:YAG.

In FIG. 2, a second embodiment of a laser resonator 2 according to theinvention (hereinafter also referred to in short as resonator 2) isillustrated which differs from the embodiment according to FIG. 1 inthat an arrangement or element for dissipating heat from the lasermedium 4 is provided. In this embodiment, the arrangement is formed by acarrier 10 which is connected to the mirror 6 and thus is in a thermallyconductive connection with the laser medium. The carrier 10 consists ofa material with a high thermal conductivity so that an efficientdissipation of heat from the laser medium 4 is achieved. As is apparentfrom FIG. 2, the carrier 10 in this embodiment is arranged outside ofthe resonator 2. Due to the heat dissipation from the laser medium 4 viathe carrier 10, a heating of the laser medium 4 and the saturableabsorber mirror 8 is reduced during the operation of the laser medium 4.

In FIG. 3, a third embodiment of a resonator 2 according to theinvention is illustrated which differs from the embodiment according toFIG. 2 in that the saturable absorber mirror 8 is at least partiallythermally decoupled from the laser medium 4. In the illustratedembodiment, an air gap 12 is formed between the saturable absorbermirror 8 and the laser medium 4 for decoupling the saturable absorbermirror 8 from the laser medium 4. In this manner, a direct heat transferfrom the laser medium 4 to the saturable absorber medium (saturableabsorber mirror 8) is prevented.

In this manner, an effective cooling of the saturable absorber medium isachieved or made possible so that the life span of the saturableabsorber medium is increased. If the optical path length of the laserresonator 2 is for example ≦1,000 μm, the air gap 12 can have a gapwidth of 10 to 500 μm. In order to prevent that the pump light used forpumping the laser medium 4 radiates into the saturable absorber medium,the laser medium 4 in the embodiment according to FIG. 3 has a coating13 on its side facing the absorber medium for reflecting the pump lightback into the laser medium 4.

What the embodiments according to the FIGS. 1 to 3 have in common is thebehavior which results in a mode selection, in which optical path lengthwithin the resonator 2 is dispositive. The optical path length is afunction of the temperature because during a temperature change, thegeometrical conditions in the resonator 2 change according to thecoefficients of thermal expansion of the media used and also the opticalconditions change according to the temperature dependency of therefractive index of the media. Thus, a single-mode emission of the lasercan only be achieved by adjusting the temperature of the media of theresonator 2. According to the invention, the temperature is varied untila resonator mode lies in the emission maximum of the amplificationbandwidth. In this manner, an arrangement for stabilizing the opticalpath length of the laser resonator is formed. It is required here tocreate the tuning range of the temperature change wide enough so that ata given temperature increase, a corresponding frequency change towardsthe emission maximum is achieved.

In FIG. 4, a fourth embodiment of a laser resonator 2 according to theinvention is illustrated which differs from the embodiment according toFIG. 3 in that the saturable absorber mirror 8 is arranged on a carrierelement 14, and between the carrier element 14 and the laser medium 4,two spacers 16, 16′ are arranged. According to the invention, thematerial of the spacers 16, 16′ is selected with respect to itscoefficient of thermal expansion in such a manner that during atemperature change, the optical path length of the laser resonator 2changes. Through appropriate selection of the coefficient of thermalexpansion it is therefore possible to influence the optical path lengthof the laser resonator 2 via a temperature change in the desired manner.

If in the embodiment according to FIG. 4, the heat generated in thelaser resonator 2, in particular in the laser medium 4, is dissipatedvia the carrier element 10, the spacers 16, 16′ can consist of athermally insulating material or a material with poor thermalconductivity. If generated heat in FIG. 4 has to be conducted to theleft to be dissipated in this manner, the spacers 16, 16′ can consist ofa material with high thermal conductivity.

FIG. 5 illustrates the dependency of the position of an oscillatinglaser mode within the amplification bandwidth. Here, only modes with anamplification >1, thus above the laser threshold, can oscillate. Achange of the relative position of the modes within the emissionspectrum by use of a temperature change makes it possible to induce thelaser into a pulsed operation. According to the invention, thegeneration of short pulses is carried out by using a saturable absorbermedium.

FIG. 6 illustrates, greatly schematized, a laser arrangement accordingto the invention for generating short laser pulses. The laser resonator2 is pumped here via a diode laser 18, the pump light of which iscoupled into the resonator 2 via a telescope 20. A portion of thegenerated laser radiation reaches a photodiode 26 via a beam splitter22, 24, and the photodiode generates an electric current which isproportional to the output power of the laser resonator 2. This signalis converted in a control device, such as a controller 28 into anactuating variable which influences the optical path length of the laserresonator as controlled variable. Actuating variables can be, forexample, the pump diode current and/or the pump diode temperature of thediode laser 18, the temperature at a piezo element serving as spacer(cf. reference numbers 16, 16′ in FIG. 4), or other parameters.

The invention provides a microcrystal laser by use of which, on the onehand, a monomode operation is achieved and, on the other hand, shortlaser pulses with low amplitude fluctuation can be generated.

While this invention has been described as having a preferred design, itis understood that it is capable of further modifications, and usesand/or adaptations of the invention and following in general theprinciple of the invention and including such departures from thepresent disclosure as come within the known or customary practice in theart to which the invention pertains, and as may be applied to thecentral features hereinbefore set forth, and fall within the scope ofthe invention.

What is claimed is:
 1. A microcrystal laser for generating laser pulsescomprising: a laser resonator defined by first and second spaced apartmirrors, the first minor being a saturable absorber; a laser mediumlocated between the first and second minors, and wherein an air gap isprovided located within the laser resonator at a position between thefirst mirror and the laser medium and wherein second minor functions asthe output coupler for the laser resonator and wherein the gain mediumis excited by directing pump light through the first minor and wherein acoating is provided on one of (1) the side of the laser medium facingthe saturable absorber or (2) the side of the saturable absorber facingthe gain medium, said coating for reflecting pump light back into thegain medium; a support for carrying the first mirror and including aspacer coupled to said laser medium, said spacer spanning the air gap ina direction of an optical path length of the laser resonator, saidspacer having a coefficient of thermal expansion selected so that whenthe microcrystal laser is heated, the spacer will expand at a rate thatcauses the width of the air gap to vary in a manner to adjust theoptical path length of the laser resonator; and a control device formonitoring the output of the microcrystal laser and in response theretovarying the temperature of the microcrystal laser in order to vary theoptical path length of the laser resonator.
 2. The laser recited inclaim 1 wherein the coefficient of thermal expansion of the spacer ishigher than the coefficient of thermal expansion of the saturableabsorber.
 3. The laser recited in claim 1 further including a controldevice for controlling the optical path length of the laser resonator asa function of the output power of one of the microcrystal laser, thepulse repetition rate, and the wavelength of the laser mode.
 4. Thelaser recited in claim 1, wherein the geometrical path length of thelaser resonator is less than 500 μm.
 5. The laser recited in claim 1,wherein the laser medium includes a side facing the saturable absorbermedium; and wherein said coating for reflecting pump light back into thelaser medium is provided on the side facing the saturable absorbermedium.